An amount of images handled by individual users has been increasing with recent widespread broadband communication, high capacity recording media, widely-used digital cameras, image scanners, and the like. Accordingly, the need for an effective image retrieval method is increasing.
Conventional image retrieval methods include a retrieval method based on semantic features such as keyword sets assigned to image data sets. Besides the above, many methods have been proposed in the fields of database management and information retrieval.
For example, there is a method in which a user assigns a keyword set to an image and then inputs the keyword to retrieve a desired image. However, this method requires the user to assign the keyword set or the like to the images. Therefore, there is a problem that the appropriateness of the keyword significantly affects retrieval performance.
In order to solve the above problem, a technology of automatically providing keywords to images has been expected. One approach of this technology is a method for acquiring keywords by analyzing Web data from which image data is acquired (see Patent Document 1, for example). The method described in Patent Document 1 is limited to the case where keywords are assigned to images acquired from the Web, and thus is not a versatile method.
Moreover, there have been proposed various methods for receiving an input of a sample image as a query image (retrieval requesting image), and then for presenting an image similar to the query image (Content-Based Image Retrieval: CBIR) (see Non-patent Document 1, for example). The method described in Non-patent Document 1 or the like calculates low-level image features, such as color, texture, and shape, for each image. Then, the method described in Non-patent Document 1 or the like retrieves a similar image by regarding a distance calculated between the image features of the respective images as a distance between the images. In other words, the method described in Non-patent Document 1 or the like presents an image having the image feature most similar to that of the query image inputted by the user, as the similar image.    Patent Document 1: Japanese Patent Application Publication No. 2006-277169    Non-patent Document 1: A. A. Goodrum, “Image information retrieval: An overview of current research”, Inf. Sci., vol. 3, pp. 63-66, 2000.